For reasons of saving fuel and minimizing pollutant emissions, in suitable operating states in modern motor vehicles, the internal combustion engine is shut off. To enable driving despite a engine start-stop function, a short starting process of the internal combustion engine and an immediate closed linkage in the transmission of the vehicle is necessary.
In prior art automatic transmissions of vehicles or, for example, in automatic gearboxes made with wet-running disk clutches, the clutches are adequately supplied with hydraulic fluid by a fluid supply only when the internal combustion engine is running. When a closed linkage is established in this transmission, first the clutch play must be overcome and the clutch or clutches must be completely closed by raising the fluid pressure. The equalization of the play of the clutch and its connection to the closed linkage are achieved by routing an amount of fluid into the piston space of the hydraulically triggerable clutches which are to be connected, the amount of fluid being made available by the fluid supply. When the vehicle starts off again after the internal combustion engine had been shut off, potentially a plurality of switching elements in the transmission are open and must be closed. As a result of this closing, an unduly long time interval can transpire until a completely closed linkage in the transmission is in fact established.
This disadvantage is resolved in prior art transmissions with a hydraulic pump driven by an electric motor and in which the delivery volume of the pump is independent of the speed of the internal combustion engine. In the absence of pressure supply by the main pump of the transmission in the hydraulic system, this hydraulic pump produces at least a pressure by which the clutch play can be equalized. The hydraulic pump which can be driven by the electric motor, however, leads to deterioration of the efficiency of the transmission and to increased cost of the transmission. Moreover, not inconsiderable additional installation space in motor vehicles and engineering effort are required for linking to the hydraulic system of the transmission and to an electrical control system.
DE 10 2006 014 756 A1 furthermore shows and describes a device for storage and for the pulsed release of an amount of hydraulic fluid for the transmission of a motor vehicle. A storage housing can be dynamically connected to the transmission for replacement of the amount of fluid and is bordered by the storage space and a movable limiting means formed as a piston. The piston is arranged so that it can be displaced within the storage space and is triggered by an actuator. The piston is furthermore held by a lock in its pretensioned position. For this purpose, the lock has catches keeping the piston in a pretensioned position against a first energy storage device formed as a compression spring. The lock can be moved by the actuator into a release position for the piston. The amount of fluid in this case is delivered pulsed into the transmission made, in particular, as an automatic transmission.
With such devices, essentially, amounts of fluid matched to the demand of a specific consumer can be set in a defined manner for obtaining a practical engine start-stop function for automatic transmissions. The prior art devices have at least a complex structure or require not inconsiderable installation space which, as a rule, only rarely is available in vehicles in the desired scope.
Furthermore, the lock and the actuator of these devices are generally often unnecessarily complex since the number of their parts is not exactly minimized with reference to representing the necessary functions.